The use of blanks of lithium silicate glass ceramic in dental technology for the production of dental restorations has been proven because of their light-optical characteristics and their strength and biocompatibility. Heat treatment results in a final crystallization of the glass ceramic to yield good optical qualities and sufficient chemical stability in particular. Corresponding methods are disclosed in, for example, DE 197 50 794 A1 or DE 103 36 913 B4.
To achieve a high strength and at the same time a good translucency, at least one stabilizer from the group zirconium oxide, hafnium oxide or a mixture thereof, in particular zirconium oxide, is added to the raw starting materials in the form of lithium carbonate, quartz, aluminum oxide etc., i.e., the usual starting components. Attention is drawn here, for example, to DE 10 2009 060 274 A1, WO 2012/175450 A1, WO 2012/175615 A1, WO 2013/053865 A2 or EP 2 662 342 A1. Machining of these zirconium oxide containing lithium silicates is also possible in the final crystallized state.
The publications of I. L. Denry et. al., Enhanced Chemical Strengthening of Feldspathic Dental Porcelain, J Dent Res, October 1993, pages 1429 to 1433, and R. R. Seghi et. al., Effects of Ion Exchange on Hardness and Fracture Toughness of Dental Ceramics, The International Journal of Prosthodontics, Volume 5, No. 4, 1992, pages 309 to 314, disclose studies of composite ceramics which are comprised of feldspathic glass types in which leucite precipitates may be present. To increase strength, it was proposed to replace sodium ions by lithium ions and then to replace lithium ions by potassium ions in a two-step process. Smaller ions can also be replaced by rubidium ions. This enabled an increase in strength of up to a maximum of 80% if rubidium oxide was used. Rubidium, however, has the disadvantage that the heat expansion coefficient of the ceramics is increased.
DE 30 15 529 A1 discloses a method to improve the mechanical strength of dental porcelain. In this method a restoration is coated with enamel so that there is an exchange of alkali ions in the enamel. For this purpose the restoration is immersed in a bath of melted salt at a temperature between 200° C. and the transition point of the enamel.
U.S. Pat. No. 4,784,606 A discloses a dental brace of glass, the strength of which is increased by ion exchange.
A method for increasing the hardness of a silicate glass object, such as a bottle is disclosed in DE 24 01 275 A1 the object is preferably heated to at least 370° C. and is sprayed with a pulverized mixture of alkali metal salts. This enables ion exchange which increases strength.